The present invention relates to apparatus for heating and sealing glass, and more particularly to a burner manifold with dual gas and temperature controls for use in sealing a wafer in a CRT (cathode ray tube) neck.
A CRT comprises three major sections, namely a panel, a funnel, and a neck. The neck comprises at its end remote from the panel an electron gun that is mounted on a wafer with lead wires for the gun electrodes projecting through the wafer. Surrounding the lead wires at the wafer are round portions of glass called "fillets" to provide a better seal around the lead wires.
During manufacture, the CRT is held in the vertical panel-up position, and the wafer with the gun mounted thereon is upwardly inserted into the neck. Heat from burners is then applied to the outside of the neck proximate the wafer, i.e., at the "seal plane", and the CRT and the wafer are rotated about their vertical central axes so that the neck softens, thins and then seals to the wafer. Also excess neck material that is lower than the wafer (cullet) is cut off and therefore falls away from the neck. However, the optimum temperature for softening is not the same as that for cutting. Thus a compromise temperature of about 950.degree. C. is used.
However, if for any reason the axis of the gun does not match the axis of the neck, or if the plane of the burners does not substantially match that of the wafer, then an incomplete cullet cut-off resulting in excess glass material called a "hanger" can be produced. The normal response of a production worker is to increase the temperature of the burners to achieve a better cut-off. Unfortunately, this can result in a "hot seal", where a portion of the inside of the neck glass melts and forms a sharp angle called a "reentrancy" with one or more of the fillets. Such reentrancies are stress concentration points that can result in a break in the wafer (called a "cracked stem" or "cracked seal"). Other problems with a hot seal include creating a burn hole, so that evacuation of air from the CRT is not possible during a subsequent pump-down operation, and creating an excessively thin region of the neck glass, which region can be easily broken.
The present invention overcomes the above problems.